Various processes have been known for preparing an aromatic or heteroaromatic thiol or disulfide. Among them, the conventional processes for preparing an aromatic or heteroaromatic thiol by bond cleavage in alkyl sulfide are classified into the following three processes:
(A) Process comprising reacting an aromatic alkyl sulfide with alkyl thiolate in hexamethylphosphoric triamide (abbreviation HMPA) used as a solvent
(Tetrahedron Letters 21 3099 (1980)) PA1 (Synthesis Communications 478 (1982)) ##STR1## PA1 (Org. Synth. Coll. Vol. 5 419 (1973)) PA1 (J. Am. Chem. Soc. 81 4939 (1959)) ##STR2## PA1 (Tetrahedron Letters 25 1753 (1984)) ##STR3##
(B) Process comprising reacting an aromatic alkyl sulfide with metal sodium in liquid ammonia
(C) Process comprising oxidizing an aromatic methyl sulfide with perbenzoic acid, reacting the oxide with trifluoroacetic acid, and causing triethylamine to act on the reaction mixture
However, these conventional processes pose the following drawbacks when commercially conducted.
The process (A) is defective in that the HMPA used as a solvent is not easily available and expensive and that the sodium mercaptide used as a reagent is difficult to handle in a nonaqueous system. Further problematic is the disposal of dialkyl sulfide produced as a by-product.
The process (B) is industrially difficult to carry out because of high risks involved in handling, use of an expensive metal sodium and low-yield production. The process (C) requires a number of reaction procedures and necessitates the use of an expensive, highly dangerous reaction reagent such as perbenzoic acid, trifluoroacetic acid, etc.
As described above, none of the conventional processes for preparing an aromatic or heteroaromatic thiol by bond cleavage in alkyl sulfide are satisfactory from the viewpoint of industrial application.